Ceramic coatings have been used for many years in a variety of industries. One particular use of ceramic coatings is in the field of furnace refractories. Specifically, ceramic coatings with high emissivity are applied to the interior of a furnace to improve the efficiency of the furnace.
Generally, high emissivity coatings are comprised of a refractory pigment, a high emissivity additive and a binder/suspension agent. Typical refractory pigments include zirconia, zirconia silicate, aluminum oxide, aluminum silicate, silicon oxide, etc. The high emissivity additive is typically a transition metal oxide such as chromium oxide (Cr.sub.2 O.sub.3), cobalt oxide (CoO.sub.x), ferrous oxide (Fe.sub.2 O.sub.3), and nickel oxide (NiO). In some coatings, the refractory pigment and the high emissivity additive are the same material.
The binder/suspension agent allows the coating to be applied like ordinary housepaint and withstands the anticipated use temperature. The binder/suspension agent acts like a high temperature glue and is typically an aqueous solution or suspension of silicates or phosphates.
Several characteristics of a ceramic coating determine whether it can be successfully applied to a particular substrate. Typically, thermal expansion matching, mechanical bonding, chemical bonding, and surface stress characteristics have been considered as well as factors such as high temperature properties, corrosion resistance and wear resistance. Traditionally, the key concern for coatings to be applied in variant temperature conditions was the match of thermal expansion coefficients for the coating and the substrate. The emphasis on thermal expansion teaches away from using materials with high coefficients of thermal expansion. Cost is another factor which is considered when coatings are produced. If the coating is more costly than the projected energy savings then the coating is not cost effective.
When a high emissivity coating is applied to the interior surface of a furnace, the thermal radiation properties of the refractory are enhanced thereby reducing fuel consumption and allowing increased product throughput without increasing the average furnace temperature. A high emissivity coating allows thermal energy to be directed and redirected toward the furnace working zone and thereby best utilize the thermal energy for the workpieces contained in the furnace. The coating reduces the exterior temperature of the furnace since the thermal energy is kept within the hot zone and is not lost in the insulation and through the furnace walls.
The heat treating industry has incorporated the use of high emissivity coatings in furnaces. The heat treating industry typically operates at temperatures up to 1100.degree. C. The coatings, which are typically chromium oxide based, are effective and stable at temperatures under 1100.degree. C. Unfortunately, at higher temperatures these coatings disintegrate over a relatively short period of time. Specifically, chromium oxide vaporizes at temperatures over 1100.degree. C. Moreover, other standard transition metal oxide high emissivity additions also have high vaporization characteristics as well as fluxing characteristics, whereby the entire mixture is lowered in overall melting point, preventing high temperature use for long periods of time.
The petrochemical industry is an example of an industry which utilizes temperatures up to 1650.degree. C. (3000.degree. F.) to process ethane, propane and similar hydrocarbons in a thermal-cracking process with steam to form ethylene and propylene. It is desirable to use a high emissivity coating in the thermal-cracking furnaces which can effectively withstand the higher temperatures and can save fuel through better thermal efficiency as well as evening out the hot zone (providing a more uniform furnace temperature).
High emissivity coatings are also useful in the glass industry in large glass furnaces. Moreover, high emissivity coatings are useful in large steel melting furnaces.
Therefore, it is an object of the present invention to provide a high emissivity coating which can be utilized to increase the efficiency of a furnace at temperatures above 1100.degree. C.
It is another object of the present invention to provide a high emissivity coating which is durable at temperatures above 1100.degree. C.
Further, it is an object of the present invention to provide a high emissivity coating which has a consistency similar to that of housepaint and applied in the same manner as housepaint.
It is yet another object of the present invention to provide a high emissivity coating which is economical.
Moreover, it is another object of the present invention to provide a high emissivity coating which is durable in reactive atmospheres.